hi^er high water (HHW)— The higher of the two high waters of any tidal day. 



hitler low water (HLW)— The higher of the two low waters of any tidal day. 



hydrograpliic datum— A datum used for referencing depths of water or the lieights of 

 predicted tides. See also datum. 



International Great Lakes Datum (IGLD) (1955)— Mean water level at Pointe-au-Pere, 

 Quebec, on the Gulf of St. Lawrence over the period 1941-56, from which dynamic 

 elevations tliroughout the Great Lakes region are measured. The term is often used to 

 mean the entire system of dynamic elevations rather than just the referenced water level. 



isostatic adjustment— Vertical motion of the Eartii's surface in response to changing in 

 loading: downward when increased weight is added; upward when weight is removed. 



Kappa (k)— Name of Greek letter used as the symbol for a constituent epoch when referred 

 to the local equilibrium argument and frequently taken to mean the same as local epoch. 

 See epoch (1). 



Ki (lunisolar diurnal constituent)— This constituent, with Oi , expresses the effect of the 

 Moon's decUnation. Both account for diurnal inequality and, at extremes, diurnal tides. 

 With Pi , it expresses the effect of the Sun's declination. 



K2 (lunisolar semidiurnal constituent)— This constituent modulates the amplitude and 

 frequency of M2 and S2 for the declinationai effect of tlie Moon and Sun, 

 respectively. 



long period constituent— A tidal or tidal current constituent with a period that is 

 independent of the rotation of the Earth but which depends on the orbital movement of 

 the Moon or of the Earth. The principal lunar long-period constituents have periods 

 approximating the month and half-month, and the principal solar long-period constit- 

 uents have periods approximating the year and half-year. 



low water (LW)— The minimum height reached by a falling tide. The height may be due 

 solely to the periodic tidal forces or it may have superimposed upon it the effects of 

 meteorological conditions. Use of the synonymous term, low tide, is discouraged. 



low water datum (LWD)— (1) the dynamic elevation for each of tlie Great Lakes and Lake 

 St. Clair and tlie corresponding sloping surfaces of the St. Marys, St. Clair, Detroit, 

 Niagara, and St. Lawrence Rivers to which are referred tlie deptiis shown on the 

 navigation charts and the authorized depths for navigation improvement projects. 

 Elevations of these planes are referred to IGLD (1955) and are Lake Superior (600.0 

 feet), Lakes Michigan and Huron (576.8 feet). Lake St. Clair (571.7 feet). Lake Erie 

 (568.6 feet), and Lake Ontario (242.8 feet). (2) An approximation of mean low water 

 that has been adopted as a standard reference for a limited area and is retained for an 

 indefinite period although it may differ sUghtly from a better determination of mean low 

 water from a subsequent series of observations. Used primarily for river and harbor 

 engineering. Boston Harbor low water datum is an example. 



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